This invention relates to a novel technique for increasing the number of semiconductor laser transmitters which meet a predefined wavelength specification and more particularly to a technique for changing a laser's center wavelength by adjusting its operating temperature.
Most laser-based communication systems have an associated range of acceptable operating wavelengths. That is, the system designers will specify a given wavelength of .lambda..sub.o as the center wavelength, with an acceptable range of .+-..DELTA..lambda.. In many cases, as higher and higher bit rate systems are employed (for example, &gt;1 Gb/s), the acceptable wavelength range about the center wavelength becomes increasingly smaller. This system specification is passed on to device designers and/or manufacturers who must supply lasers which satisfy the criterion at the lowest cost possible. For example, a particular high bit rate laser-based transmission system may require 1.3 .mu.m InGaAsP lasers having a center wavelength of 1308 nm, with a range of .+-.4 nm.
Because lasers as manufactured obviously have a distribution of wavelengths, a narrow system specification profoundly affects the number of manufactured devices which may be used in the system. In an exemplary manufacturing operation, perhaps less thn 25% of the manufactured lasers may meet the required wavelength specification. Although manufactured lasers which exhibit a transmitting wavelength outside of the predetermined range may be used in other applications with different wavelength requirements, the majority are often discarded as scrap. This large percentage of scrap, coupled with the cost of manufacturing lasers, leads to a significant premium in expense which must be added to the cost of the acceptable lasers. Thus, it would be desirable to increase the number of lasers which satisfy a particular wavelength specifications.
Many systems exist in the prior art for controlling the operating wavelength of a laser once the system has been installed. One exemplary arrangement is disclosed in U.S. Pat. No. 4,485,475 issued to S. F. Large et al on Nov. 27, 1984. Large et al utilizes a combination of a GRIN lens and diffraction grating to monitor the drift of a laser's output wavelength. As the output drifts beyond .+-..DELTA..lambda., a drift detector sends a control signal to a thermoelectric cooler which either raises or lowers the laser temperature to drive the wavelength back within the defined range. Such dynamic wavelength control arrangements, however, are utilized only with installed lasers which meet the wavelength specification.
Thermoelectric coolers are also utilized with laser transmitters to maintain a constant laser operating temperature, 20.degree. C. being a conventional operating temperature. In some applications, the ambient temperature may range anywhere from -50.degree. C. to +100.degree. C., yet the thermoelectric cooler must maintain the laser at 20.degree. C. with minimal variation (e.g., .+-.0.5.degree. C.). The laser itself may generate a head load on the order of 80-100 milliwatts which must also be absorbed by the thermoelectric cooler. U.S. Pat. No. 4,631,728 issued to B. S. Simons on Dec. 28, 1986 discloses an exemplary circuit arrangement for monitoring the laser's temperature and controlling the response of the thermoelectric cooler. The Simons circuit purports to exhibit improved efficiency, necessary for laser-based systems, over prior art arrangements by utilizing a pulse width modulated controller, switching transistor and network filter to supply the load current to the thermoelectric cooler.
Neither the Large et al. nor Simons arrangements, however, address the problem described above of finding a means of increasing the percentage of manufactured lasers which meet a given wavelength specification. Large et al. merely controls the operating wavelength after a laser (which presumably meets the required wavelength specification) is installed. Simons maintains an installed laser at a predetermined operating temperature, regardless of the ambient temperature.
Thus, a need remains for a method of increasing the number of system-acceptable lasers without sacrificing the required specifications, in particular the wavelength specification.